Due to the daily motion of the earth, every longitude on the earth is exactly opposite to the sun once in $24$ hours so in all the places on any one longitude the sun comes on one head in $24$ hours.
And all of those places are moderated at the same time and that time is called $12$ noon.
Time All the locations on that longitude are called local time.
Since each longitude is in front of the Sun, the local time of each is different.
The longitude of any city can be known if local time is given.